[llvm-mca][View] Improved Retire Control Unit Statistics.

RetireControlUnitStatistics now reports extra information about the ROB and the
avg/maximum number of entries consumed over the entire simulation.

Example:
  Retire Control Unit - number of cycles where we saw N instructions retired:
  [# retired], [# cycles]
   0,           109  (17.9%)
   1,           102  (16.7%)
   2,           399  (65.4%)

  Total ROB Entries:                64
  Max Used ROB Entries:             35  ( 54.7% )
  Average Used ROB Entries per cy:  32  ( 50.0% )

Documentation in llvm/docs/CommandGuide/llvmn-mca.rst has been updated to
reflect this change.

llvm-svn: 347493